1. Field of the Invention
The invention relates to a prong type resin cage incorporated in a double row roller bearing, and a double row roller bearing including a prong type resin cage.
2. Description of the Related Art
A bearing portion by which a main spindle is rotatably supported in a machine toll is required to have a high degree of stiffness for the purpose of maintaining a high degree of machining accuracy. Thus, a double row roller bearing is used. Further, in recent years, because there has been a demand for speedup of rotation of a main spindle, a double row roller bearing capable of withstanding high-speed rotation has been required.
A double row roller bearing includes an inner ring, an outer ring, and a plurality of rollers. The rollers are arranged in two rows between the inner ring and the outer ring. For example, Japanese Patent Application Publication No. 2012-102796 (JP 2012-102796 A) (refer to FIG. 3) describes a double row roller bearing including independent cages each of which holds a plurality of rollers arranged in a corresponding one of the two rows. That is, the double row roller bearing includes two cages. Each of the cages has an annular portion and a plurality of cage bars. The cage bars extend in the axial direction of the cage from one side face of the annular portion, and are located at intervals in the circumferential direction of the cage. The cages are formed in a comb-shape. Pockets in which the rollers are held are defined between the cage bars that are adjacent to each other in the circumferential direction.
Because a prong type cage has a cantilever structure in which cage bars are projected from an annular portion in the axial direction, distal end portions of the cage bars are deformable to some extent. Thus, for example, even if rolling of rollers fails to keep up with the rotation of a double row roller bearing and thus tensile force and compression force repeatedly act on the cage bars, these forces are relieved. Therefore, the cages are less prone to damages. On the other hand, in a window-type cage in which paired annular portions are connected to each other via cage bars, the cage bars are fixed to the annular portions located on the respective sides of the cage bars, and thus deformation of the cage bars is restricted. Thus, if tensile force and compression force repeatedly act on the cage bars, these forces are not easily relieved. Therefore, the window-type cage is more susceptible to damages than the prong type cage is.
The rotational speed of a main spindle of a machine tool is selected from a range from a low rotational speed to a high rotational speed (e.g. 15,000 rpm), and is rotated at various speeds. The rotational speed of a double row roller bearing and the rotational speed of a cage incorporated in the double row roller bearing are both changed depending on variations in the rotational speed of the main spindle.
In the case of a double row roller bearing that is rotated at a high rotational speed, preferably, a cage is made of resin and “outer ring guide” is adopted, that is, the cage is positioned in the radial direction by the inner peripheral face of an outer ring. The outer peripheral face of an annular portion of the cage serves as a guide face that is guided by the inner peripheral face of the outer ring. That is, the cage is rotated in the circumferential direction while being guided at the outer peripheral face of the annular portion, by the inner peripheral face of the outer ring.
However, if the cage is rotated at a high speed such as 15,000 rpm, the cage is deformed so as to be enlarged in the radial direction by a centrifugal force. Thus, it is necessary to form a radial clearance between the outer peripheral face (guide face) of the annular portion of the cage and the inner peripheral face of the outer ring, the radial clearance containing beforehand an amount of deformation caused by the centrifugal force. If a large radial clearance is set in view of an amount of deformation caused by high-speed rotation, the radial clearance is unnecessarily large when the cage is rotated at a low rotational speed because the amount of deformation at a low rotational speed is smaller than that at a high rotational speed. Thus, the position of the cage in the radial direction is not fixed at a low rotational speed, and, for example, the contact between the cage and the inner peripheral face of the outer ring is repeated irregularly or regularly, resulting in noise generation.